Adds POST/DELETE handlers for /Users/{userId}/FavoriteItems/{itemId} and
/Users/{userId}/Items/{itemId}/Rating. A shared resolveAnnotated helper
probes album/artist/media file (mirroring getItem's order) and 404s before
writing if the user lacks access to the album/song's library; artists are
exempt since they span multiple libraries. Ratings are halved coming in
(Jellyfin 0-10 -> Navidrome 0-5) to match the doubling in dto.UserData.
Also teaches MockMediaFileRepo and MockArtistRepo's SetStar/SetRating (and
MockAlbumRepo's, previously a no-op) to actually mutate the backing data so
write-back can be asserted in tests.
d28adb2676